As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential and have long cycle lifespan and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is increasing recently, research into electric vehicles and hybrid electric vehicles that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, a nickel-metal hydride (Ni-MH) secondary battery is mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively carried out and some of the lithium secondary batteries are commercially available.
As a cathode active material of a lithium secondary battery, a lithium-containing cobalt oxide (LiCoO2) is mainly used. In addition, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, and the like, and lithium-containing nickel oxides such as LiNiO2 is also being considered.
Among these cathode active materials, LiCoO2 has excellent lifespan characteristics and high charge and discharge efficiency and thus is most widely used. However, LiCoO2 has low safety at high temperature and cobalt used as a raw material thereof is expensive due to limited reserves, and thus, LiCoO2 has limitation in price competitiveness.
Lithium manganese oxides such as LiMnO2, LiMn2O4, and the like have high thermal stability and are inexpensive and easy to synthesize, while having low capacity, poor high-temperature characteristics and low conductivity.
In addition, LiNiO2-based cathode active materials are relatively cheap and exhibit excellent battery characteristics such as high discharge capacity. However, swelling due to occurrence of cation mixing in which some Li cation sites are substituted with Ni cations, deterioration of high-rate discharge characteristics, and rapid phase transition of a crystal structure according to volumetric change through repeated charge and discharge occur. In addition, when exposed to air and moisture, stability of such cathode active materials is dramatically reduced.
To address these problems, use of a lithium oxide containing Ni—Mn in a mix ratio of 1:1 or Ni—Co—Mn in a mix ratio of 1:1:1 as a cathode active material has been tried and research thereinto has been underway.
Batteries including a cathode active material prepared by mixing Ni, Co, or Mn exhibit superior cycle and capacity characteristics to a battery manufactured using a cathode active material prepared using each of the transition metals (Ni, Co, and Mn). Even in this case, however, when used for extended periods of time, cycle characteristics are rapidly deteriorated and problems such as swelling by gas generation due to cation mixing, deterioration of high-rate discharge characteristics, and the like have yet to be sufficiently addressed.